1. Field of the Invention
The present invention relates to a power conversion control device. Specifically, the present invention relates to a power conversion control device that can be used for power conversion control of a motor generator used in a hybrid car or the like as a main driving engine.
2. Description of the Related Art
A motor generator drives a steering system as a main driving engine of a hybrid car (power running mode). In addition, the motor generator generates electric power by using the driving force from the system when the hybrid car is running using a gasoline engine (including when braking) that is a supplemental driving engine, and charges a battery (power regeneration mode). When the motor generator is used as an electric motor, electric power supplied from the battery is stepped up (i.e boosted) by a converter. The electric power is then converted to alternating current voltage by an inverter, and the motor is driven. In power regeneration mode, the electric power supplied from the power generator is stepped down by the converter, and the battery is charged. The converter (power conversion circuit) converts electric power that is given and received between the battery and the inverter.
A known converter includes an upper-arm switching element, a lower-arm switching element, diodes, an inductor, and a capacitor. The diodes are connected in reverse parallel with each switching elements. The inductor is connected between the connection point of the switching elements and a positive terminal of the battery. The capacitor is connected in parallel with the series connection body comprising of the lower-arm switching element and the upper-arm switching element.
In the converter, during power running, energy is stored in the inductor by the lower-arm switching element being turned ON. The energy in the inductor is discharged through the upper-arm diode by the lower-arm switching element being turned OFF. On the other hand, during regeneration, energy is stored in the inductor by the upper-arm switching element being turned ON. The energy in the inductor is discharged by of the upper-arm switching element being turned OFF.
The output voltage of the converter is controlled by the upper-arm switching element and the lower-arm switching element that is alternately turned OFF, regardless of whether the motor generator is in power running mode or power regeneration mode.
Here, when the ON operation is switched to another switching element, a dead time is provided during which both switching elements are turned OFF. When the direction of a current flowing through the inductor (also referred to as reactance) is reversed, error attributed to the dead time increases.
Therefore, for example, as in JP-A-2011-223674, variably setting carrier frequency in a pulse-width modulation (PWM) process for generating operating signals of the switching elements based on the size of the current flowing through the inductor is proposed. However, in this technology, in a state in which the current flowing through the inductor is small and the direction is reversed, electric power conversion efficiency may decrease as a result of the current outputted from the battery to the capacitor side returning from the capacitor side to the battery side during a single ON/OFF cycle of the switching elements. Therefore, a power conversion control device is desired that does not reduce electric power conversion efficiency.